1. Field of the Invention
Embodiments of the present disclosure relate generally to normal relay emulators and more particularly to normal relay emulator control circuits.
2. Discussion of Related Art
A normal relay is a relay that maintains a set position unless the relay is energized. More specifically, a normal relay will be in either the open or closed state (referred to as the “normal” state), when the coil within the relay is not energized. When the coil is energized, the relay will enter into the “non-normal” state. For example, a normally open relay provides an open circuit unless the relay is energized, in which case the relay will be closed. Similarly, a normally closed relay provides a closed circuit unless the relay is energized, in which case, the relay will be open. Once power to the coil of a normal relay is interrupted, the relay will return to the “normal” position (e.g., either open or closed). As will be appreciated, however, normal relays require a certain amount of current when energized. More particularly, a constant supply of current is required to keep the coil within the relay energized. Furthermore, the current draw of a normal relay increases with the size of the normal relay. As such, larger relays (e.g., those needing several watts of power to energize the coils) continually draw current during operation. Accordingly, a normal relay will be a constant drain on power circuitry (e.g., batteries, generators, alternators, or the like).
In order to reduce the drain on circuits using normal relays, bi-stable relays may sometimes be substituted for a normal relay. A bi-stable relay is a relay that remains in its last state when power to the relay is shut off. Said differently, a bi-stable relay can be changed from either open to closed or closed to open, by temporarily energizing the relay. The bi-stable relay will then remain in this state when power to the relay is interrupted and therefore does not draw current (or draws a small amount of current) while the relay is not changing position. As such, a bi-stable relay has a lower quiescent current requirement than does a normal relay. However, as will be appreciated, when power to the circuit including the bi-stable relay is interrupted, the bi-stable relay will remain in its last position. As a result, a bi-stable relay cannot be used to replace a normal relay in circuits where the relay must be switched to a known position when the supply of power is interrupted.
In order to address this, control circuits are available that cause a bi-stable relay to enter a known position when power to the control circuit is interrupted. These control circuits use an energy storage device (e.g., capacitor) to store energy, which is used to energize the bi-stable relay when power to the control circuit is interrupted. Accordingly, the bi-stable relay will enter a known state after the supply of power is interrupted. However, for larger bi-stable relays, a large energy storage device is required in order to be able to supply enough current to operate the bi-stable relay when power to the control circuit is interrupted. In many applications, there is insufficient space to accommodate such a large capacitor. Furthermore, the necessity of a large capacitor adds increased costs to the control circuit.
This problem is further exacerbated where the control circuit is operated from a lower voltage power supply (e.g., 12 volts, 9 volts, or the like). In such systems, charging a large capacitor to the nominal input voltage (e.g., 12 volts) will not produce enough energy to actuate the bi-stable relay and cause the bi-stable relay to change state in the absence of input power.
Thus, there is a need for a relay that returns to a known position when power is interrupted and has a low current draw during periods where the relay is not changing position. Furthermore, there is a need for such a relay that can operate on lower voltage systems and still actuate a large relay.